In recent years, there has been an increasing need for batteries that supply power to wearable equipment such as a smart watch, a wearable health monitor, and smart glasses, and there has accordingly been a need for small lithium-ion secondary batteries that are safer and have a high capacity.
Examples of a battery installed in wearable equipment include a small laminate lithium-ion secondary battery and a cylindrical lithium-ion secondary battery. The cylindrical lithium-ion secondary battery is required to prevent bursting thereof due to an increase in internal pressure of the battery during abnormal functioning (for example, when an active material or an electrolytic solution is decomposed to produce gas).
With regard to a cylindrical lithium-ion battery having a typical size (for example, a size of 18650), PTL 1 discloses a method of deforming a valve member toward a through-hole as a result of pressure being applied to a portion formed of a ring including the valve member and the through-hole for cleaving the valve member when the internal pressure of the battery increases and causing the valve member to cleave when the pressure reaches a predetermined pressure.
Protrusions are formed so as to extend toward the inside of the through-hole of the ring that supports the valve member. In this case, stress is concentrated on the valve member at the tip of the protrusions when the internal pressure increases. Accordingly, the valve member cleaves at a normal pressure with certainty, the gas produced in the inside of the battery can thereby be released, and bursting of the battery can be avoided. Consequently, operating pressure can be decreased even when the area of the valve member is small.
With regard to a prismatic lithium-ion battery, PTL 2 discloses a structure for cleavage in which a protrusion is disposed so as to extend in the direction perpendicular to a three-layered laminate cover and is caused to stick in a surface of the laminate cover.